Fluidization has been successfully applied to a large number of unit operations as well as to catalytic reactions. A conventional fluidized bed is characterized by very rapid solids mixing, vertically, horizontally, and radially, so that the bed remains homogeneous in particle size, product composition, and temperature. However, in some industrial or chemical processes this very high overall mixing is not desirable since it reduces the efficiency of the process. In those particular cases it would be preferable to attain such uniformity and homogeneity within small layered segments of the bed. This can be achieved with a controllable plug flow of solids from laYer to layer to attain the desired stepped size distribution, product composition, or temperature gradient.
In the past such small layered bed segments have been manifested as a superposed series of fluidized beds, one above the other, with solids overflowing into downcomer tubes leading to a lower bed. A disadvantage of this stacked bed arrangement is that the fluidizing gas bearing entrained material flows upwardly from one bed through the support grid of the following bed above with the result that all the upper stages or beds are enriched in the finest particle sizes while the lower stages are enriched in the coarsest particle sizes. This in turn results in an internal gradient which may be detrimental to the reaction process, particularly if the particles constitute a catalyst. In order to reduce this inevitable size gradient phenomenon, the beds must be separated by at least the transport disengaging height (TDH) distance, and this leads to undesirably tall reaction vessels if a large number of fluidized bed stages is necessary.
A further disadvantage of reaction vessels comprising vertically stacked fluidized beds is that pressure drop limitations caused by a backup of material in downcomers preclude operation with deep beds (which may be desired to provide the requisite processing time in the bed) unless there are commensurate large bed to bed spacings. Moreover, downcomers between superposed beds are often unstable with respect to solids flow and are prone to pluggage if the material is not free flowing.
Another disadvantage of vertically stacked fluidized beds is that the grids supporting higher beds are subject to pluggage by the solids entrained in the fluidizing gas from the bed below. Also, stacked beds generally exhibit large pressure drops so that high pumping energy is required for the fluidizing gas.